In cellular base stations, satellite communication systems as well as other communication and broadcasting systems of today, it is often desirable to amplify multiple radio frequency (RF) channels simultaneously in the same power amplifier instead of using a dedicated power amplifier for each channel. However, when using one and the same power amplifier for the simultaneous amplification of multiple RF channels spread across a fairly wide bandwidth, a high degree of linearity is required so that the phases and amplitudes of all the signal components are preserved in the amplification process.
If the linearity is inadequate, the simultaneously amplified channels cross-modulate, causing interference in these and other channels. The non-linearities manifest themselves as cross-modulation of different components of the signal, leading to leakage of signal energy to undesired channels. In addition, the spectra of the signal components are normally broadened, causing additional interference within the channels or in other channels.
In addition to linearity, one of the most important properties of a power amplifier is efficiency. The efficiency must be kept high in order to reduce the need for cooling as well as the overall power consumption, and to increase the lifetime of the amplifier.
Consequently, the problem of enhancing the linearity must be solved without sacrificing the amplifier efficiency.
A common way of increasing the efficiency of an RF power amplifier is to use the Doherty principle as described and developed in references [1-7]. FIG. 1 is a schematic block diagram of a conventional Doherty amplifier. The Doherty amplifier 100 is a so-called composite amplifier, which in its basic form comprises two sub-amplifier stages, a main amplifier 110 and an auxiliary amplifier 120. The auxiliary amplifier 120 is connected directly to the load 130, and the main amplifier 110 is connected to the load through an impedance inverter 140, usually in the form of a quarter wavelength transmission line or an equivalent lumped network
At low output levels, only the main amplifier 110 is active. In this region, the main amplifier 110 sees a higher (transformed) load impedance than the impedance at peak power, which results in increased efficiency. The input drive arrangement 150 of the auxiliary amplifier 120 is configured with a non-linear drive function f2(x) such that when the output level climbs over the so-called transition point (usually half the maximum output voltage), the auxiliary amplifier kicks in, starting to drive current into the load 130. Through the impedance-inverting action of the quarter wave transmission line 140, the effective impedance at the output of the main amplifier 110 is reduced such that the main amplifier is kept at a constant maximum voltage above the transition point. The key action of the Doherty amplifier occurs in the region where the auxiliary amplifier 120 is active, and the main amplifier 110 is close to its maximum voltage condition, with high overall efficiency as a result.
However, conventional Doherty amplifiers only provide satisfactory linear performance and efficiency in a relatively narrow frequency band. The quarter wavelength impedance inverter provides a phase shift of 90 degrees only at a single frequency, and the output of a practical Doherty amplifier will be distorted at frequencies away from this so-called center frequency because of a reflection of the output current of the auxiliary amplifier at the impedance inverter. Losses in the transistors, the impedance inverter and the DC feed networks may also give rise to unexpected distortion. In addition to these sources of distortion, Doherty amplifiers will in practice always suffer from non-linearities caused by non-linear output parasitic elements such as parasitic conductances and capacitances, commonly referred to as parasitics.
It is generally known that the non-linearities encountered in Doherty amplifiers are strongly frequency-dependent. The non-linearities will manifest themselves both as (modulated) harmonic overtones and intermodulation products. The intermodulation products are the most severe for communication systems since the harmonic overtones can be filtered away before the signal reaches the antenna. The intermodulation products on the other hand appear right among the desired signals and can generally not be filtered away before transmission. The complex frequency dependency makes it very difficult to compensate for the non-linear intermodulation products by using pre-distortion. Simple pre-distortion techniques can not compensate for these non-linearities. In fact, a very complex and hence expensive pre-distorter implemented with digital signal processing (DSP) techniques will be required. Such a complex pre-distorter is furthermore difficult to adjust properly and will generally not optimize the efficiency.
Consequently, there is a general demand for an improved technique of compensating for non-linearities in a composite amplifier.